My Wii Remote had automatically de-synced after lying around for 1/2 year (with dead batteries inside) and I could not get it to re-sync. Neither the red sync button nor the “add remotes” in the home button menu worked. New batteries did not help.
What fixed it:[br]
I removed the batteries for 3+ minutes, then re-synced the Wii Remote with the red sync button while in System Menu.